The decay constant β of π-conjugated molecular wire corresponds to the efficiency of electron tunneling through the molecule. We have calculated β values of intramolecular magnetic exchange interactions by density functional theory for organic bis(nitronyl nitroxide)s that have several molecular wire units, namely, oligo-p-phenylene (OPP), oligophenylene ethynylene (OPE), oligophenylene vinylene (OPV), oligophenylene imine (OPI), bicyclo[1.1.1]pentane oligomer (BCP), β,meso,β-triply fused porphyrin oligomer (TFP), and meso,meso-linked porphyrin oligomer (MLP). For OPP, OPE, OPV, OPI, BCP, TFP, and MLP, β was calculated to be 0.41, 0.25, 0.22, 0.32, 0.77, 0.03, and 0.65 Å–1, respectively. The obtained β values are in good agreement with the experimental value of the molecular tunneling conductance, suggesting similarity of the mechanism between exchange interaction and molecular conductance. This method can be used to estimate the efficiency of electron tunneling through the molecular wire.